This invention concerns variable speed gas turbine engines of a type that can be especially useful for driving electric generators or vehicles and in other applications where it is preferred to have a moderate power output, small size and high response speed. The efficiencies of variable speed turbines vary according to a number of variables, including engine speed. In prior art gas turbines of this type, when the turbine speed decreases and the power output remains unchanged, such as in the case of where a vehicle travels up a slope, the compressor speed will rise, its power will increase, and much more fluid than necessary will be supplied to the turbine. The engine consequently becomes "overcooled", the cycle temperature drops, and contraction of metal parts occurs. As a result, turbine power output decreases, and efficiency is reduced.
When turbine speed remains unchanged, and the power output decreases, such as in the case of where a vehicle travels down a slope, compressor speed decreases to a great extent and the turbine experiences a shortage of fluid. The engine thus becomes "overheated," which poses risks to turbine engine components due to excessive metal overheating and expansion.
During overcooling, the compressor turbine has an excess of power that floods the turbine with fluid. During overheating, there is a shortage of power at the compressor turbine, and the turbine receives less fluid than it needs, which leads to overheating. Thus, temperature is a critical parameter to control in engines of this type.
Both phenomena have been counteracted by controlling fluid flow to the compressor turbine or by controlling fluid flow to the turbine. In both cases, such control is accompanied by losses. For example, the prior art counter-rotating gas turbine engine shown at RU 2,082,894 uses control vanes to control fluid flow to the turbine.
This prior art gas turbine engine is advantageous because it does not have controllable stator vanes of the turbine. This lack of controllable stator vanes makes the engine more economical, especially during variable operation conditions. This engine also has better acceleration characteristics. This gas turbine engine does, however, have drawbacks, because it is controlled by the compressor stator vanes. Controlling the engine in this way make the engine susceptible to temperature swings yield temperatures outside safe limits, especially at low speeds. This is a serious disadvantage in partial load applications, such as in vehicle applications where the partial load characteristics are critical. In addition, controlling fluid flow using mechanical devices such as vanes or throttles is inefficient because such devices yield high losses. Even if the control method efficiently keeps the temperature within safe limits, the overall efficiency of the engine is still rather low in view of the losses.
In addition to efficiency concerns, the mechanical device used to control gas flow will have high inertia. High inertia reduces engine responsiveness to operating parameters. High inertia is dangerous in this circumstance because the control device will not have time to increase fluid flow to the turbine if the temperature rises suddenly.
This disadvantage is eliminated in a gas turbine engine according to the invention as described below.